This dissertation explores the role of religion in the origins of the Cold War from 1880 to 1948. Building on David Foglesong’s research into the role of religion in shaping American missionaries, businesspeople, and public intellectuals’ perceptions of Russia, as well as Andrew Preston’s insights into the Franklin D. Roosevelt administration’s use of religious tropes to justify intervention against Nazi Germany from 1939 to 1945, this project focuses on the White House and US State Department’s efforts to manage diplomatic tensions and public controversies surrounding religious repression in Russia during the origins of the Cold War from 1880 to 1948. The central finding of this project is that during the period from 1933 to 1945, President Franklin D. Roosevelt and his personal allies such as Joseph Davies sought to minimize popular and official criticisms of Soviet religious policies as a part of Roosevelt’s program of pragmatic cooperation with the USSR. Eventually, anti-communist officials in the State Department managed to undermine Roosevelt’s public relations program in order to justify a more confrontational approach to the Soviet regime. Roosevelt’s poor health, growing personal isolation, and neglect of personal relationships with American Catholic leaders after 1943, as well as his failure to create a bureaucracy committed to his vision of post-war cooperation, meant that after his death religion could be used by anti-communists in their campaign to denigrate the Soviet Union. To gain popular support for its containment and roll-back strategies, the Truman administration called for a worldwide Christian crusade to eradicate atheistic communism. By shedding light on how well the Roosevelt administration was able to overcome US-Russian religious tensions, this project supports the “missed opportunities” thesis that the Cold War was not inevitable. It also stands as an example of a growing body of scholarly research linking religion, diplomacy, and US foreign relations.